To: I finally have one regret in my life, that I can’t ever take back… Me: I understand your pain and why you feel the way you do. Not everyone does. You did the best with what you had and no one could say you didn’t.. It died, but by the sounds of it, that was going to happen sooner or later. Death is not a scary place, in all probability it was an escape which it welcomed, because the harsh world it died in, was no longer a problem and it would be out of pain and it understood, that for it, there was a different kind of future, just the same as when we eventually die. Death is the final solution and comes to us all, sooner or later.
I have had a number of big dogs which reached their used by date – they could no longer walk, or s*** or piss on their own and it was beyond me to help them any further. Like you I have regrets and Guilt that I could have done more.
My advice to you is to be proud of what you achieved. Do not hold yourself in any way responsible for the eventual final outcome which occurred. You did the best you could with what you had and nothing or nobody could ask for more.
What goes around, comes around and eventually they might find themselves needing help which does not come. Such is life.